Light emitting diodes (LEDs) have many advantages over conventional light sources, such as incandescent, halogen and fluorescent lamps. These advantages include longer operating life, lower power consumption and smaller size. Consequently, conventional light sources are increasingly being replaced with LEDs in traditional lighting applications. As an example, LEDs are currently being used in flashlights, traffic signal lights, automotive taillights and display devices.
Among the various packages for LEDs, an LED package of interest is the plastic leaded chip carrier (PLCC) package for a surface mount LED. Surface mount LEDs in PLCC packages may be used, for example, in automotive interior display devices, electronic signs and signals, and electrical equipment.
A concern with the current process for producing PLCC packages is the problem of thermal expansion between different materials used in PLCC packages. Because materials expand and contact differently, thermal stress is created between different materials. A coefficient of thermal expansion (CTE) is often used to characterize how different materials expand or contract with changes in temperature.
Thermal stress may initiate mini cracks along interfacial surfaces. Thermal stress may also cause de-lamination between a die and a lead frame for example. Thermal cycling conditions (i.e. repeated changes in temperature) that occur during normal operation may propagate mini cracks to the extent a die that is attached to a lead frame may be lifted from the lead frame.
Silicone is commonly used as a material to encapsulate a light source in a PLCC because it is soft and pliable. Because silicone is soft and pliable, it is often used to reduce cracks in a PLCC package. However, silicone is not as useful as other materials for use as an optical lens due to its softness.